Myocardial infarction (MI) is a primary cause of morbidity and mortality worldwide. Although reperfusion therapy improves post-MI survival, the rapid restoration of oxygenated blood flow to the heart elicits oxidative stress that further damages tissue, known as ischemia-reperfusion (IR) injury, exacerbating one?s risk for developing heart failure (HF). Central to IR injury are mitochondria, which play key roles in cardiomyocyte Ca2+ buffering and production of energy and reactive oxygen species (ROS). Upon IR, mitochondrial Ca2+ overload and ROS trigger mitochondrial permeability transition (mPT) pore opening, which promotes cell death. While many studies highlight the critical role of mPT in IR injury and cardioprotection, many molecular pathways leading to altered mitochondrial Ca2+ and ROS (key upstream regulators of mPT) remain poorly defined. In recent work, we discovered that a heart- and muscle- enriched long, ?non-coding? RNA actually does encode for a highly-conserved single-pass transmembrane micro- protein that localizes to the inner mitochondrial membrane; we named this protein ?Mitoregulin? (Mtln). In gain- and loss-of-function studies, we found that Mtln ?supercharges? mitochondria by increasing their 1) respiratory super- complex levels, 2) respiratory efficiencies, and 3) Ca2+ retention capacities, while reducing ROS. In this grant, we propose Aims to address two central hypotheses: 1) that Mtln protects against cardiac IR injury by delaying Ca2+- and ROS-triggered mPT, and 2) that human genetic variation linked to Mtln expression associate with HF patient outcomes. Our overarching goal is to translationally link Mtln with cardiac IR injury and HF outcomes to justify a need for future studies to continue defining Mtln?s precise modes of action. In Aim 1, we will define the physiologic and molecular functions of Mtln in cardiomyocytes by evaluating the acute effects of modulating Mtln expression on mitochondrial respiration, Ca2+ and ROS, as well as supercomplex levels in cultured rodent cardiomyocytes. We will also define Mtln domains and protein interactions necessary for its function. In Aim 2, we will elucidate a role for Mtln in cell and rodent models of IR injury and HF through gain- and loss-of-function studies to evaluate the impact of Mtln modulation on cardiomyocyte death induced by simulated IR, as well as on responses to cardiac IR injury and ensuing HF in mice. In Aim 3, we will work to establish links among genetic variants, cardiac Mtln expression, and HF patient outcomes, focusing on the biological and genetic relevance of a 36-bp 3?UTR deletion variant (Mtln-3?UTRdel) that is present in 12% of the general population and is known to be linked to decreased Mtln mRNA levels in heart tissues. We will identify RNA-binding proteins that control Mtln expression and assess the impact of the 3?UTRdel variant on this regulation and on Mtln protein levels in human hearts. We will also test available DNA samples linked to clinical data in patient cohorts to assess the association of Mtln-3?UTRdel genotypes with HF incidence and outcomes (e.g. mortality and arrhythmic events). These multi-disciplinary studies have significant potential to 1) transform our knowledge of basic mitochondrial biology, 2) determine if Mtln represents a viable target for HF therapy, and 3) reveal new genetic markers that could translate towards improved identification and treatment of high-risk HF patients.